"Anonymous believes... that pursuing this direction is a sad mistake on your behalf. Not only does it reveal the fact that you do not seem to understand the present-day political and technological reality, we also take this as a serious declaration of war from yourself, the UK government, to us, Anonymous, the people," the group said in a statement (PDF) on Thursday.

"Our advice to you, the UK government, is to take this statement as a serious warning from the citizens of the world," it added.

The Office of Cyber Security (OCS), the part of the Cabinet Office that co-ordinates the UK's computer defence and attack efforts, said that intelligence service GCHQ was monitoring the situation through the GovCertUK computer emergency response team.

"These arrests are an operational matter for the Metropolitan Police Service," an OCS spokesman told ZDNet UK. "[The government] continues to monitor the situation and the potential threat to UK government websites from groups of this nature."

GCHQ, through the CESG information assurance agency and GovCertUK, provides government departments with guidance on how to protect against, detect and mitigate various types of cyberattack. It also acts as a single point for reports of security incidents on the government network.

Computer Misuse Act

In its statement, the group characterised the UK arrests as "exactly like arresting somebody for attending a peaceful demonstration in their hometown" and said that DDoS attacks are not akin to hacking. It also rejected the idea that DDoS attacks are crimes under the UK's Computer Misuse Act.

In response, William Malcolm, a computer law expert and senior associate at Pinsent Masons solicitors, argued that DDoS is prohibited under the legislation. "The Computer Misuse Act was effectively amended to take in denial-of-service attacks, by amendments to the Police and Justice Bill 2006," he said.

Sam Jardine, an associate for law firm Eversheds, noted that the Police and Justice Bill amendments came into force in 2008. He said he was not aware of any prosecutions for DDoS in the UK, although there is case law for denial-of-service attacks.

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